The object of this research is to further explore the idea that ventilatory work-rate (W) is a consistent index of respiratory controller output in the regulation of alveolar C02 tension in humans. The specific aims of this work are: 1) to determine the influence of changes in the mechanical resistance and compliance of the respiratory system upon the steady and dynamic responses of the workrate controller in C02 regulation; 2) to examine the effect of changes in the background of respiratory stimuli of exercise, hypoxia, and hyperthermia upon the dynamic and steady state responses of the W-C02 controller; and 3) to determine those circumstances in which the W-C02 controller selects a tidal volume-frequency combination compatible with optimal control using workrate or force minimization criteria. The transient and steady ventilatory workrate responses of human subjects to step increases and decreases in inspired C02 will be determined in the normal, resting control condition; the effects of added external viscous and elastic loads to the system, of hyperthermia, and of mild exercise will be determined by comparison with the resting control. In addition to the dynamic and steady response data, breath-by- breath measures of pulmonary resistance and compliance, and inspiratory, expiratory, and total cycle workrate done on the lungs will be determined by computer analysis of the esophogeal pressure and respiratory airflow signals. With mechanical parameters of the respiratory system known for each subject, prediction of the optimal breathing pattern using various energy expenditure criteria will be made in each situation for comparison with observed responses.